This invention relates to the field of steam turbine buckets and, particularly, to machining of buckets mounted in the turbine.
Steam turbines generally have annular rows of turbine buckets that are mounted on a rotor. Each row of turbine buckets is arranged around a disc wheel mounted on the rotor. Typically each turbine bucket has a blade section, and a upper and lower shroud sections. The upper shroud is generally referred to as the xe2x80x9ccoverxe2x80x9d of the bucket. The buckets are arranged annularly around the outer periphery of the wheel. The wheel is mounted on the turbine shaft. Several rows of turbine wheels are arranged on the turbine shaft. Each wheel is separated by some predefined distance, e.g. approximately five inches (15 cm), to allow for turbine vanes that are arranged between the rows turbine buckets.
The covers of the buckets are at the outer perimeter of the turbine wheel and bucket assembly. The covers are adjacent the stationary turbine casing. To prevent steam passing over the buckets from leaking over the casing and into the casing, a seal is formed between the casing and covers of the buckets. As part of this seal, sealing teeth are machined onto the upper surface of the covers after the buckets have been assembled onto the wheel. The sealing teeth on the covers are aligned with similarly configured teeth on spill strips of the turbine casing. The non-contact engagement of the sealing teeth on the covers and the teeth on the spill strips prevent steam from leaking past the buckets and into the casing, thereby improving the efficiency of the turbine unit.
The machining of the bucket covers can create metal burs on the covers, including burs that extend into the gaps between adjacent bucket covers. The standard past practice for machining away burs has involved removal of the buckets from the turbine wheel, which requires disassembly of the turbine. After the burs are ground down on the removed bucket, the buckets and turbine are reassembled. This prior bur removal process is extremely time consuming and expensive.
There are occasions when the turbine buckets are machined and repaired after they have been assembled on a disc, and the disc has been mounted on the shaft of the turbine. For example, veneer-sealing teeth are often machined into the ICVs, after their buckets have been mounted on a wheel and the wheel mount on a shaft. Machining the veneer-sealing teeth into the ICVs after the buckets have been assembled on a wheel ensures that the teeth on each cover line up and are aligned with their opposite teeth on the stationary spill strips mounted on the turbine housing.
Machining veneer-sealing teeth often leaves metal burs on ICVs. Some of these metal burs are on the upper surface of the covers and will extend into the gaps between the covers and the spill strips, and other burs may protrude from the sides of the covers and interfere with the interlocking of a cover with its adjacent covers. Shims have been inserted between bucket covers to reduce burs from rolling into the gap between covers.
However, it is difficult to access the gap between turbine covers to insert and remove shims after the buckets have been mounted on a wheel and the wheel mounted on the turbine shaft. It is especially difficult to access the steep-angle interval cover buckets (ICVs) that have been developed to improve steam turbine efficiency. Nevertheless, ICVs and other types of bucket covers do require additional machining after their buckets have been assembled on the wheel and shims are useful for reducing burs.
If the burs on ICVs are substantial they can affect the response characteristics of the turbine bucket to vibration. In particular, large burs on ICVs have been shown to produce substantial resonance frequency shifts in the axial and torsion vibration modes of a bucket. Burs may shift the resonance vibration frequencies of a bucket by more than 10 percent (10%) from the expected resonance frequency for the bucket. Accordingly, the resonance frequency shifts caused by the burs on the ICVs can render inaccurate the expected resonance frequencies for buckets.
Turbine designers rely on the expected resonance frequencies of a bucket to, for example, select the number of upstream nozzles to be adjacent the row of buckets. If a designer properly understands the resonance frequency of the turbine buckets, then the number of upstream nozzles may be selected to minimize the resident frequencies in the buckets. If burs offset the expected resonance frequency of the bucket, the actual vibration resonance of the bucket with burs may unintentionally coincide with vibrations induced by the upstream nozzles and rotating buckets as steam flows from the nozzles to the buckets. If vibrations induced by the steam have frequencies at or near the resonance frequencies of the bucket, then excessive vibration may be induced in the bucket that will cause the bucket to prematurely fail.
There is a long felt need for better tools to machine burs from buckets and use shims to reduce bur formation, especially from the cover of buckets. Such tools,should make the insertion and removal of shims expeditious and inexpensive.
A tool has been developed to spread apart the covers of adjacent buckets mount on a turbine wheel. The tool may be applied when the wheels are assembled on a turbine shaft. By separating the covers, shims can be inserted into and removed from the covers.
In one embodiment the invention is a bucket spreading tool for separating covers of adjacent turbine buckets, wherein the tool includes: an arm for extending a head of the tool between adjacent turbine wheels and for positioning the head between the adjacent buckets of a wheel, and the head has an attachment to an end of the arm and a forward portion having a front side surface shaped to engage a first bucket of said adjacent buckets, and a rear side surface shaped to engage a second bucket of said adjacent buckets.
The spreading tool may further include a front surface of the head that is cupped to abut a convex surface of the first bucket, and a rear surface of the head is rounded to pivot against the second bucket. Further, the spreading tool head may include a slot between its front and rear side surfaces, wherein said slot is alignable with gap between the covers of the adjacent turbine buckets.
The bucket spreading tool of the first embodiment may have a tool head that is separable from the arm, and tool head height that is no greater than three inches which is less than the distance between the adjacent turbine wheels. Further the tool head may be formed of a soft metallic material.
In a second embodiment the invention is a bucket spreading tool for separating covers of adjacent turbine buckets having: an arm for extending a head of the tool between adjacent turbine wheels and for positioning the head between the adjacent buckets of a wheel; the head including an attachment to an end of the arm and a forward portion having a front side surface shaped to engage a first bucket of said adjacent buckets, and a rear side surface shaped to engage a second bucket of said adjacent buckets, wherein the front side surface is concave to mate with a convex airfoil surface of the first bucket, and the rear side surface of the head is convex to pivot against a concave airfoil surface of the second bucket.